The correct choice is (d) 5.625A
To elaborate: 1.When 20v source acting alone: current source is replaced by open circuit. Req= 5+3=8Ω and Current I= 20/8=2.5A
2. When 5A source acting alone: 20v source is replaced by a short circuit. By current division rule, I3= 25/8=3.125A
Total current through I3= 2.5+3.125=5.625A.